1. Field of the Invention
This invention relates to improved treated carbon fibers, electrodes formed from such treated carbon fibers, and a process for making such treated carbon fibers. More particularly, this invention relates to mesophase carbon fibers treated to increase the density of exposed edges thereon, electrodes formed from such treated mesophase carbon fibers characterized by an increased density of active sites formed from such exposed edges, and a process for treating mesophase carbon fibers to increase the density of active sites on electrodes formed from such treated fibers.
2. Description of the Related Art
Graphitized carbon has been used in the formation of electrodes and catalysts for many electrochemical and chemical applications. In general, however, only the edge sites of the graphite structure function as active sites responsible for the desired chemical or electrochemical reactions, with the basal plane of the graphite structure remaining relatively inert in the reaction environment. It is, therefore, desirable to increase the edge site density of graphite materials to achieve high performance. However, for most graphitized materials, the percentage of the edge sites to basal plane sites in graphitized carbon is limited due to the nature of the graphite structure and material processing.
In practical applications, carbon electrodes are usually formed from graphite powders which are either pressed into a shaped electrode or coated on a current collector with a binder. The carbon electrode structure formed by pressing the graphite powders has a very low density of active or edge sites, while the carbon electrode structure formed using a current collector and a binder adds weight and cost to the system.
Carbon fibers have also been used in the construction of carbon electrode structures. Such carbon fibers have higher conductivity than corresponding carbon powder structures, making them a superior choice in some electrochemical applications. Furthermore, such carbon fibers exhibit high mechanical strength and superior flexibility which enable them to be fabricated into various shapes and forms to fit a particular application.
Nishino et al. U.S. Pat. No. 4,562,511 describes the use of carbon fibers as an electrode in the formation of a capacitor; while Takahashi et al. U.S. Pat. No. 4,980,250 and Kano et al., in "Carbon Fiber as a Negative Electrode in Lithium Secondary Cells", J. Electrochemical Soc., Vol. 139, No. 12 (December 1992) at pages 3397-3404 both describe the use of PAN based carbon fibers in the formation of a composite electrode for a lithium secondary cell at low current density. The use of chopped vapor-grown carbon whiskers as a negative electrode in a lithium battery is discussed by Zaghib et al. in "Optimization of Electrochemical Properties of Graphite Whisker for Use as Negative Electrode in Lithium Ion Rechargeable Batteries by Control of Whisker Dimensions", The Electrochemical Society Proceedings Volume 94-28, at pages 121-135.
While the use of carbon fibers in the formation of carbon electrodes has its advantages, many conventional carbon fibers have an onion-like structure with edge sites only exposed at the ends of the individual fibers, i.e., resulting in a low density of active sites on such fibers.
Recently, a new type of carbon fiber, know as a mesophase carbon fiber, has become available commercially. In a mesophase carbon fiber, the graphite-like single layer atomic sheets are arranged radially around the axis of the fiber, i.e., to radiate from the axis to the surface of the fiber. The surface of the fiber is then coated with a layer of hard carbon which encapsulates or covers the edges of such radial sheets. FIGS. 1A-1D illustrate such fibers, which are generally indicated, respectively at arrows 2a-2d, with lines 10a, 10b, 10c, and 10d respectively showing the radial disposition of the interfaces between the graphite-like sheets comprising the fiber, resulting in the formation of radial edges 12a-12d in the respective fibers. Such fibers, as available commercially, are almost inert for chemical reactions, except at the end edges of the individual fibers, because the radial edges 12a-12d referred to above are formed with a hard carbon outer layer or coating, as shown at 20 (in an exaggerated scale for illustrative purposes only) in FIGS. 1A-1D. Hard carbon shell 20 covers or sheaths the respective radial edges 12a-12d so that such covered edges cannot function as active sites in an electrode formed using such fibers.
This hard shell on the mesophase fiber is apparently formed during the spinning of the fiber. When the fiber is spun through the spinnerette, the surface of the fiber is subject to a large shear stress because it is in direct contact with the extruder and moving at a very high speed, resulting in the formation of an amorphous structure. After carbonization and graphitization, this skin becomes a continuous dense amorphous carbon layer with a low density of edge sites, similar to glassy carbon or the basal plane of graphite. Such mesophase carbon fibers conventionally find application in the construction of composite materials because of their excellent mechanical properties.
Because of the dense amorphous nature of the hard shell on the mesophase fiber, early tests on the use of such fibers in the construction of electrodes for rechargeable cells indicated that the rate of charging and discharging was very low, i.e., rendering the long continuous mesophase fibers not useful in that form. However, it was found that such mesophase fibers could be useful if they were ground up into small segments having a length of from about 30 to 50 micrometers (.mu.m). In essence, the chopped or ground fibers were then treated as regular carbon powder, requiring the use of conventional binders and current collectors to function as an electrode. Norio Takami et al., in "Rechargeable Lithium-Ion Cells Using Graphitized Mesophase-Pitch-Based Carbon Fiber Anode", J. Electrochemical Soc. 142 (1995) at page 2564, describes such uses of ground mesophase fibers in the formation of carbon electrodes for rechargeable cells.
It would be desirable if a carbon fiber could be developed which would have a high density of exposed edges and which would exhibit low corrosion, yet have the mechanical strength and flexibility of conventional carbon fibers which would permit fabrication of carbon electrodes therefrom in various shapes and characterized by a high density of active sites formed from such exposed edges.